1. Field of the Invention
The present invention relates to an apparatus and a method for pre-compensating an asymmetrical picture so that in a projection system for displaying a picture, a normal picture is provided regardless of the projection angle of a projection lens with respect to a screen.
2. Description of the Prior Art
Generally, a picture display system for displaying a color picture is divided broadly into direct-view picture display systems represented by CRTs (Cathode Ray Tubes), and projection systems for displaying a picture (hereinafter, referred to as "projectors") represented by LCDs (Liquid Crystal Displays). Since CRTs are restricted in its size due to its inherent structure, they cannot provide large-scale color screens. On the other hand, LCDs are able to provide large-scale color screens with slim and light structures, but they cause an optical loss.
Therefore, a projector has been widely used. This projector breaks up a color picture signal according to color into RGB light signals, and optically adjusts the RGB signal broken up according to color on the basis of the components of the color signal. The picture signal which is optically adjusted, is magnified via a projection lens and is displayed on a relatively large-scale screen.
FIGS. 1 and 2 are respectively views for showing the arrangement of a projection lens with respect to a screen and the shape of a picture projected onto the screen in the projector when the picture projected onto the screen is normal. As shown in FIGS. 1 and 2, a center axis 30 is a line connecting a point on a first traverse axis 22 which crosses at right angles a midpoint point 21A of a first longitudinal axis 21 on a screen 20, with a center point 11A of a projection lens 11 of a projector 10. When center axis 30 forms an approximate right angle with the surface of screen 20, a first projection distance 41 which ranges from center point 11A of projection lens 11 to an upper end of the picture, i.e., screen 20, becomes equal to a second projection distance 42 which ranges from center point 11A of projection lens 11 to a lower end of the picture, i.e., screen 20. Also, a third distance 23 which ranges from center axis 30 to the upper end of the picture, i.e., screen 20, becomes equal to a fourth distance 24 which ranges from center axis 30 to the lower end of the picture, i.e., screen 20, so that a width 25 of the upper end of the picture becomes approximately equal to a width 26 of the lower end of the picture. As a result, as a whole, the picture is projected in the shape of a rectangle onto screen 20 without keystoning or trapezoidal distortion.
FIGS. 3 and 4 are respectively views for showing an arrangement of the projection lens with respect to the screen and the shape of the picture projected onto the screen in the projector when the projection lens has an upward projection angle with respect to the screen. As shown in FIGS. 3 and 4, when projection lens 11 of projector 10 is placed below a first plane (not shown) which vertically traverses first longitudinal axis 22 with reference to the surface of screen 20, a center axis 30A has an upward projection angle with reference to the first plane. At this time, a first projection distance 41A is different from a second projection distance 42A, and the picture projected onto screen 20 has the shape of a reverse trapezoid in that a width 25A of the upper end thereof is wider than a width 26A of the lower end thereof.
FIGS. 5 and 6 are respectively views for showing an arrangement of the projection lens with respect to the screen and the shape of the picture projected onto the screen in the projector when the projection lens has an downward projection angle with respect to the screen. As shown in FIGS. 5 and 6, when projection lens 11 of projector 10 is placed above a first plane (not shown) which vertically traverses first longitudinal axis 22 with reference to the surface of screen 20, a center axis 30A has a downward projection angle with reference to the first plane. At this time, a first projection distance 41B is different from a second projection distance 42B, and the picture projected onto screen 20 has the shape of a trapezoid in that a width 25B of the upper end thereof is smaller than a width 26B of the lower end thereof, contrary to the picture in FIG. 4.
As described above, when the projection lens of the projector is arranged at an upward or downward projection angle with respect to the screen, the magnified picture that is projected onto screen 20 is distorted due to the keystoning or the trapezoidal error. In the end, the distorted picture will inevitably irritate a user, so pre-compensation for an asymmetrical or distorted picture that is projected onto the screen due to occurrence of the keystoning, is required.
For example, U.S. Pat. No. 5,355,188 discloses an apparatus and a method for compensating or correcting the trapezoidal error on the screen in the projector. In the disclosed apparatus and method, the trapezoidal error is eliminated by maintaining an optical center of a field lens on the optical axis of the projection lens.
As another example, U.S. Pat. No. 5,283,602 discloses an apparatus for compensating the trapezoidal error on the screen in the projector. In the disclosed apparatus, an optical path diversifying mirror is arranged at an inclined angle of 45.degree. with respect to an optical path of a light and is also arranged so that it can be moved in a direction parallel to the optical path toward or away from a projected object, thereby the image is projected upwardly or downwardly without the occurrence of the trapezoidal distortion on the screen.
However, in order to solve the trapezoidal distortion on the screen, the aforementioned configurations take an optical or a mechanical approach, so their performance or structural flexibility are restricted.